feature/485768 page conditions

package.json

@@ -40,7 +40,7 @@
   },
   "license": "SEE LICENSE IN LICENSE",
   "dependencies": {
-    "@defra/forms-model": "^3.0.393",
+    "@defra/forms-model": "^3.0.395",
     "@elastic/ecs-pino-format": "^1.5.0",
     "@hapi/boom": "^10.0.1",
     "@hapi/catbox": "^12.1.1",

src/server/plugins/engine/helpers.ts

@@ -1,4 +1,4 @@
-import { ControllerPath } from '@defra/forms-model'
+import { ControllerPath, Engine } from '@defra/forms-model'
 import Boom from '@hapi/boom'
 import { type ResponseToolkit } from '@hapi/hapi'
 import { format, parseISO } from 'date-fns'
@@ -155,6 +155,11 @@ export function findPage(model: FormModel | undefined, path?: string) {
 }
 
 export function getStartPath(model?: FormModel) {
+  if (model?.engine === Engine.V2) {
+    const startPath = normalisePath(model.def.pages.at(0)?.path)
+    return startPath ? `/${startPath}` : ControllerPath.Start
+  }
+
   const startPath = normalisePath(model?.def.startPage)
   return startPath ? `/${startPath}` : ControllerPath.Start
 }

src/server/plugins/engine/models/FormModel.ts

@@ -2,6 +2,7 @@ import {
   ConditionsModel,
   ControllerPath,
   ControllerType,
+  Engine,
   formDefinitionSchema,
   hasRepeater,
   type ConditionWrapper,
@@ -34,10 +35,12 @@ import {
 import { FormAction } from '~/src/server/routes/types.js'
 import { merge } from '~/src/server/services/cacheService.js'
 
+/**
+ * Responsible for instantiating the {@link PageControllerClass} and condition context from a form JSON
+ */
 export class FormModel {
-  /**
-   * Responsible for instantiating the {@link PageControllerClass} and condition context from a form JSON
-   */
+  /** The runtime engine that should be used */
+  engine?: Engine
 
   /** the entire form JSON as an object */
   def: FormDefinition
@@ -78,6 +81,7 @@ export class FormModel {
       ]
     })
 
+    this.engine = def.engine
     this.def = def
     this.lists = def.lists
     this.sections = def.sections
@@ -228,27 +232,16 @@ export class FormModel {
     // Find start page
     let nextPage = findPage(this, startPath)
 
+    this.initialiseContext(context)
+
     // Walk form pages from start
     while (nextPage) {
-      const { collection, pageDef } = nextPage
-
       // Add page to context
       context.relevantPages.push(nextPage)
 
-      // Skip evaluation state for repeater pages
-      if (!hasRepeater(pageDef)) {
-        Object.assign(
-          context.evaluationState,
-          collection.getContextValueFromState(context.state)
-        )
-      }
+      this.assignEvaluationState(context, nextPage)
 
-      // Copy relevant state by expected keys
-      for (const key of nextPage.keys) {
-        if (typeof context.state[key] !== 'undefined') {
-          context.relevantState[key] = context.state[key]
-        }
-      }
+      this.assignRelevantState(context, nextPage)
 
       // Stop at current page
       if (nextPage.path === currentPath) {
@@ -263,6 +256,49 @@ export class FormModel {
     context = validateFormState(request, page, context)
 
     // Add paths for navigation
+    this.assignPaths(context)
+
+    return context
+  }
+
+  private initialiseContext(context: FormContext) {
+    // For the V2 engine, we need to initialise `evaluationState` to null
+    // for all keys. This is because the current condition evaluation
+    // library (eval-expr) will throw if an expression uses a key that is undefined.
+    if (this.engine === Engine.V2) {
+      for (const page of this.pages) {
+        for (const key of page.keys) {
+          context.evaluationState[key] = null
+        }
+      }
+    }
+  }
+
+  private assignEvaluationState(
+    context: FormContext,
+    page: PageControllerClass
+  ) {
+    const { collection, pageDef } = page
+    // Skip evaluation state for repeater pages
+
+    if (!hasRepeater(pageDef)) {
+      Object.assign(
+        context.evaluationState,
+        collection.getContextValueFromState(context.state)
+      )
+    }
+  }
+
+  private assignRelevantState(context: FormContext, page: PageControllerClass) {
+    // Copy relevant state by expected keys
+    for (const key of page.keys) {
+      if (typeof context.state[key] !== 'undefined') {
+        context.relevantState[key] = context.state[key]
+      }
+    }
+  }
+
+  private assignPaths(context: FormContext) {
     for (const { keys, path } of context.relevantPages) {
       context.paths.push(path)
 
@@ -275,8 +311,6 @@ export class FormModel {
         break
       }
     }
-
-    return context
   }
 }
 

src/server/plugins/engine/pageControllers/PageController.ts

@@ -18,6 +18,7 @@ import {
   normalisePath
 } from '~/src/server/plugins/engine/helpers.js'
 import { type FormModel } from '~/src/server/plugins/engine/models/index.js'
+import { type ExecutableCondition } from '~/src/server/plugins/engine/models/types.js'
 import {
   type FormContext,
   type PageViewModelBase
@@ -39,6 +40,7 @@ export class PageController {
   pageDef: Page
   title: string
   section?: Section
+  condition?: ExecutableCondition
   collection?: ComponentCollection
   viewName = 'index'
 
@@ -55,6 +57,11 @@ export class PageController {
     this.section = model.sections.find(
       (section) => section.name === pageDef.section
     )
+
+    // Resolve condition
+    if (pageDef.condition) {
+      this.condition = model.conditions[pageDef.condition]
+    }
   }
 
   get path() {